This invention relates to current amplifiers in general and more particularly to a high-gain current amplifier employing MESFETS or MOSFETS.
As one can ascertain, amplifiers having wide bandwidths in excess of 100 mHz with low amplitude distortion and stable gain have been extremely important in the field of monolithic microwave integrated circuits (MMIC).
The prior art has thoroughly investigated such amplifiers and many examples of typical prior art amplifiers are known. A major breakthrough in such an amplifier configuration was described in an article entitled "A New Wide-Band Amplifier Technique" by Barrie Gilbert published in the IEEE, Journal of Solid State Circuits, Vol. SC-3, No. 4, December 1986, pages 353-365. This amplifier is based on two common circuits which were combined to produce the new amplifier configuration. The first circuit is the conventional differential pair which was widely used as a multiplier configuration. The second circuit is a current source which is employed in practically every linear IC. The article describes the combination of such circuits and in FIG. 3 show the new circuit structure. The article gives an extensive analysis of the principles of operation as well as the advantages of using the circuit.
Particularly, if reference is made to Page 363 of the article there is shown a photo-micrograph of an integrated gain-cell amplifier. The description of the amplifier as well as the operating characteristics of the amplifier are given. As indicated, such amplifiers were investigated in the prior art. The increasing use of MOS technology for the implementation of analog functions which incorporate amplifiers lead researchers to the development of various MOS building blocks such as filters, voltage references, operational amplifiers and multipliers. A conventional building block that has long been used in bipolar circuits is the 4-quadrant analog multiplier which employs differential amplifiers. Analog multiplies with performance adequate for microwave signal processing have been described and reported for use in MOS technology.
Reference is made to a paper entitled "A Four-Quadrant NMOS Analog Multiplier" which was published in the Proceedings of the IEEE, Journal of Solid State Circuits, Vol. SC-17 No. 6, December 1982 by David C. Soo, et al. This article describes circuitry which implements a 4-quadrant analog multiplier fabricated in NMOS technology. The circuit approach is based on the square law characteristic of the MOS transistor and results in a multiplier with performance approaching bipolar circuits. As can be seen, that article also cites an article by B. Gilbert, the author indicated above, and entitled "The High Performance Monolithic Multiplier Using Active Feedback" which was published in the IEEE Journal of Solid State Circuits, Vol. SC-9, pages 364-373, Dec. 1974.
The Soo article shows a differential FET pair which is fabricated from NMOS technology where the differential pair is utilized in cascaded configurations to provide an MOS-quadrant analog current multiplier which is shown in a simplified schematic in FIG. 3, page 1175 of the article.
In spite of the strides made by the prior art there is still required a high-gain amplifier circuit which desirably employs MESFETS or MOSFETS.
The amplifier according to this invention provides improved gain characteristics whereby the amplifier exhibits increased gain due to increased depth of modulation of "inner pair" tail current.
The amplifier is an open-loop amplifier which can be employed in many applications and is especially adapted to be integrated utilizing MMIC techniques.